Target tussues respond to hormones only if specific receptors are present on or within their cells. Alterations of the hormone-receptor axis may then modify the response of target tissues (i.e. hormonally responsive mammary carcinoma) to various hormones and serve as a means for controlling the growth and function of such tissues. We have shown that growth hormone induces the prolactin receptor in murine liver by acting at a site distal to the induced receptor. By making mice deficient in essential fatty acids, both the intraperitoneal and mammary gland fat are markedly decreased. These deficient animals have lowered levels of prolactin receptors and are unable to induce the receptors by the aforementioned hormone injections. Prostaglandins have also been shown to be required for the maintenance and induction of the prolactin receptor in vivo. These studies indicate that specific fatty acids modulate the functionality of prolactin receptors, probably by altering prostaglandin synthesis. The response of target tissues to various hormones may therefore be modified by the type and quantity of fat within the diet.